Does anyone have experience with designing and installing 900' deep bores with a double u-bend in parallel configuration ?
Regarding your inquiry "Does anyone have experience with designing and installing 900' deep bores with a double u-bend in parallel configuration ?" I do not have direct experience with loops in boreholes this deep. However I would expect this is possible in spite of the so-called pressure ratings on various DR schedules of HDPE pipe. The reason is that the posted pressure ratings are derived from very conservative ASTM ratings derived from empirical calculations that are not relevant to borehole conditions; they reference conditions for horizontal pipe construction as used in the transmission of oil and other fluids with partial filling of fluid and uneven pressure such as that from a sharp rock impacting the pipe. I presented a PowerPoint presentation a while back at an IGSHPA conference about this subject and the need for pipe manufacturers, IGSHPA and other stakeholders to address meaningful research to determine what HDPE pipe will actually tolerate for deeper boreholes without constricting or otherwise compromising the integrity of a u-bend assembly. I have attached a pdf of the presentation for your review, and a spread sheet describing the pressure gradient of water vs. various grout compositions. This is compared against the ASTM ratings of DR9 and DR11 pipe, both HDPE 4710 and 3408. Another issue I have with the ASTM ratings is that it does not account for the pipe size. I would expect that with smaller diameter pipe it should tolerate more external pressure at depth, but as noted in my presentation the external pressure has to also overcome the pressure resistance of water within the pipe - which does not want to compress!
From my own experience, 600' boreholes present no problem to the integrity of 1.25" DR11 pipe for example. I found references of successful installations in Sweden to 200 meters (650 feet) using thinner wall DR17 pipe (unsure of the nominal pipe size).
As the pressure ratings are about 25% of the ASTM rated maximum pressure tolerance, without water on the inside of the pipe, this suggests 900' borehole loop installations should not be a problem. To be safe maybe you could convince the client to drill a test loop installation and run a plumb bob test as shown in the presentation; if the bob runs all the way to total depth without interference you should be ok. You could than get more value out of the test hole by running a TC test (which you will likely need anyways). The thermal conductivity result will be the same regardless if you have one or two loops as you are not changing the geology.
Not certain what your specific project requirements are but have found from simulations on large closed loop ground heat exchanger simulations that little performance gains are realized using two u-bend circuits in a borehole, other than slightly reduced borehole resistance due to more exposed pipe in the borehole (more contact area for heat transfer). But typically this is substantially offset due to higher installation cost of two loops in a borehole. I could see with more flow paths the pressure drop would be reduced if you are fighting space limitations for the installation of the ground heat exchanger.
As urban installations typically have limited area for vertical loop installation I think the industry is going to see the need to determine what if any depth limitations apply to HDPE pipe more and more.
Regardless of whether the pipe can withstand the installation conditions, there may be implications from a product support / warranty point of view. Your best bet would probably be to speak with the pipe manufacturer for recommendations.
Someone in the engineering division at PPI would be another great resource - http://plasticpipe.org/ppi-staff.html.
My thanks to all of you for your insight. Most helpful!
Marco and all,
But, while Ryan is correct to have caution, there is also a big opportunity with "dry wells" (at least around here) which are effectively "free" to use for GHP, and those are often 600'-1200'. Lisa is exactly on the mark with doing the analysis, and thus the correct grout must be used to assure balancing pressure. I would add that long term considerations are also an issue -- like is this an area where the grout is going to fully dry out? If so, then balancing pressure from the grout may not be reliable (although I am not sure about that -- consult a geologist maybe!).
I think there is also the issue of advancing the field, and on that front I hope we do experiment with these bores ... at least where they have already been drilled and are thus "free". And, definitely please report back to all what is learned.
I highly recommend reviewing the following and performing the calculation. It is from the 2015 ASHRAE Applications Handbook, Chapter 34 page 27.
Pressure Considerations In Deeper Vertical Boreholes. Vertical systems have been designed and installed to depths greater than 400 feet (121.9 m) with mixed results. More field data is needed to extend this range without prominent warning. Special attention should be given to the effect of both the internal and external fluid(s) differential pressure on the HDPE pipe regardless of the specified u-bend type or insertion depth.
In most cases, the only design compensation for increased hydrostatic pressure on the u-bend assembly at these depths was to specify heavier walled HDPE pipe, e.g. DR-9 (Dimension Ratio). Below 350 feet (106.7m), in a borehole full of fluid, pipe collapse is more probable than burst due to the fact that thermal grout is denser than water. This results in an imbalance of the hydrostatic head in the annulus of the borehole versus the hydrostatic pressure exerted by the fluid inside of the pipe.
Internal Working Pressure (IWP) and External Pressure Resistance (EPR) of HDPE material decreases with time of exposure and elevated temperature. The 10 hour IWP and EPR for HDPE 3406/3408 and 4710 at 73.4oF (23oC) for DR-11 and DR-9 are shown in Table X4. The designer should check the manufacturer’s material specifications for all buried piping. Should the lower pipe DR (thicker wall) be selected, the EPR pressure increases at the expense of an increase in friction loss at any given flow. HDPE material selection, DR selection, grout selection, and the short term use of additional pressure (air or fluid) on the u-bend assembly may be remedies to deal with excessive differential pressure on the u-bend assembly.
Recent developments in clay-based thermally enhanced grout include the addition of graphite along with, or in place of, silica sand. Graphite-based-grout can have a thermal conductivity of up to 1.60 Btu/h•ft•oF (2.77 W/m-K). In addition, graphite based grout has a significantly lower density and viscosity than clay/silica sand mixtures at a given thermal conductivity (Table 5). The graphite-based products are more expensive than clay/silica sand thermally enhanced product. However, economics, geology, and site conditions may warrant the additional expense. The designer/engineer should contact the grout manufacturer for additional information. In the long term, the annular fluid in a properly grouted borehole is designed to “gel” using a clay-based grout, or harden (set) using a cementatitious grout, thus mitigating a portion of the differential pressure. In general, the maximum working time for most grouts is 30 to 45 minutes dependent on a number of factors such as make-up water temperature, borehole temperature, make-up water chemistry, etc. After this amount of time the grout will begin to gel or set and become un-pumpable. Note that grouting is not required in some jurisdictions and not addressed in others; this has given rise to substituting a manufactured fill material, which may be acceptable in some cases.
In deeper boreholes it is especially important for the designer to be aware of conditions that may lead to pipe failure and design accordingly. Equation 19 may be used to calculate differential pressure between the internal and external pressures on the HDPE u-bend assembly at any depth :
Pd = 0.052 x (D2 – D1) x Depth (ft) (19)
(Pd = 0.00981 x (D2 – D1) x Depth (Meter)) (20)
Pd = Pressure differential in psi (kPa)
D1 = Density of the internal fluid in pounds per gallon (ppg) (Kg/M3)
D2 = Density of the external fluid in ppg (Kg/M3)
Depth is measured in net vertical feet (net vertical meters)
(.052 is the pressure gradient for a fluid with a density of 1 pound per gallon one foot high.)
Terry- thanks for sharing this information! John
We regularly install SDR11 loops to 150 - 200 m and in some instances beyond, with thermal grouts in the UK and Europe. We might lighten the mix to circa 1.8 W/mk (5:1 mix) but with the diminishing return on ever increasing thermal conductivity this strikes a reasonably good balance between deeper boreholes and not inhibiting energy flow if site space is limited.
In Sweden they have installed loops to far greater depths and in Switzerland, they have popped them into 800 m but these were, as far as I am aware, generally in water filled boreholes and so the issue does not arise as the water pressure inside and out are equal during installation.
Pipe roundness also plays an important roll as loops that have been drawn tightly into coils can have an ovality to them and so for deep installations a good quality loop is important. Ovality in a loop at depth and with high density grout is more susceptible to collapse than for a very well rounded pipe.
We would normally have the loop filled with water and if very deep, put it under pressure during grouting until the grout has set. It is a bit of a conundrum for bentonite based grouts which set to a plastic consistency. Does this behave like a very very thick liquid or does it become self supporting against the formation and the loop pipe thereby mitigating the risk of loop collapse once the grout has gone from liquid to plastic? A research subject potentially... Is Chuck reading this :-)....
As mentioned in other posts graphite eases the issue of grout density.
Finally, as Terry mentions, double loops are not the silver bullet. We would normally see perhaps a 10% - 15% decrease in loop requirements or a slightly better EWT with a double loop but it is generally a tool for us for head loss control with the added bonus of a slight improvement in performance. They can be a good option if the load is very choppy i.e. from heating to cooling and back again over short periods of time because of the reduced borehole resistance as Terry also notes, as it enables the borehole to cope with the changing loads but still not much better than perhaps 15% overall improvement.
Hope this helps...
Since pipe ovality, collapse, etc. were mentioned, I thought you may be interested in this:
Wow, I knew it would make a difference from all of the collapse resistance tests we used to carry out on well casings and screens many years ago but the difference is quite staggering. Your article does confirm the self supporting properties once set so that's good...
Thanks for posting this, as this information needs to be disseminated to industry and not many people get Geoutlook over here !